Given the task definition and input, reply with output. Based on the information present in the given passage, you need to write a correct answer to the given question. A correct answer is one that correctly and completely answers the question. Do not write answers that address the question partially or incorrectly. The correct answer must be contained in the given paragraph.

Paragraph- Sent 1: You just got done with a long run.
Sent 2: You are gasping for air.
Sent 3: Why does your body react this way?
Sent 4: What is the purpose of breathing?
Sent 5: All the cells of your body need oxygen to work properly.
Sent 6: Your bodys circulatory system works with the respiratory system to deliver the oxygen.
Sent 7: Your blood carries red blood cells.
Sent 8: The main job of red blood cells is to carry oxygen throughout your body.
Sent 9: The red blood cells get oxygen in the lungs.
Sent 10: The lungs are the main organs of the respiratory system.
Sent 11: The respiratory system is the body system that takes in oxygen.
Sent 12: It then releases carbon dioxide back to the atmosphere.
Sent 13: The carbon dioxide is the waste material from the cells. 
Question: What waste material do our cells release into the atmosphere?.

CO2.